1. Field of the Invention (Technical Field)
The present invention relates to solar collector panels and the methods of making the same.
2. Background Art
Low-cost/high-performance solar collectors are needed to make solar thermal power competitive with other power sources. Incorporation of mirrors into commercially viable panels is the key to low-cost, high-performance solar collectors. In the 1970's and 1980's and with the financial support of the U.S. government, substantial development and test activities were directed toward the development of glass reflector designs for central receiver heliostats, parabolic trough collectors, and point focus concentrators (“solar collectors”). However, many problems surfaced with a majority of the solar collectors, such as the formation of dimples due to adhesion forces exerted on the glass, and the seepage of water between the bonded mirror and support resulting in silver corrosion of the mirror and consequent loss of solar reflectivity.
Glass-foam core mirrors were developed by the Jet Propulsion Laboratory (JPL). At JPL glass mirrors were mechanically deformed and bonded to a foamed glass support that had been ground to a specified contour. To minimize debonding of the glass mirror to the support, materials with similar thermal expansion coefficients were used. Argoud, M. J., 1980, “Test Bed Concentrator Mirrors,” Proceedings of the First Semiannual Distributed Receiver Systems Program Review, DOE/JPL-1060-33, Pasadena, Calif. Steel-substrate supports were used in the McDonnell Douglas dish concentrator. Stone, K. W. et al., 1993, “Status of Glass Reflector Technology for Heliostats and Concentrators,” Proceedings of the 28th Intersociety Energy Conversion Engineering Conference, Atlanta, Ga. This design entailed bonding glass mirrors to a steel sheet, which in turn was supported by a stretch-formed or stamped steel backup structure (like the support found in a car hood). A similar type of support, but with rib supports stretch-formed or stamped to the desired curvature, were used by Acurex in their Innovative Concentrator design and by Solar Kinetics, Inc. (SKI) on the Shenandoah dishes. However, both the Accurex and the Solar Kinetics concentrators used reflective film technology rather than glass mirrors. Overly, et al., 1985, “Innovative Point Focus Solar Design, Task 1, 2a, Topical Report,” DOE-AL/23711-1, Albuquerque, N. Mex., and Saydah et al., 1983, “Final Report on Test of STEP Shenandoah Parabolic Dish Solar Collector Quadrant Facility,” SAND82-7153, Sandia National Laboratories, Albuquerque, N. Mex. Lastly, fiberglass supports formed over a mandrel have been investigated by Kansas Structural (Gill, S. R., Plunkett, J. D., 1997, “Fabrication of Four Focusing Solar Collector Segments of Widely Differing Geometries From Fiber-Reinforced Polymer Honeycomb Composite Panels,” Final Report Submitted to Sandia National Laboratories, Albuquerque, N. Mex.) and McDonnell Douglas (NREL Final Report, 1998, “Solar Thermal Component Manufacturing for Near-Term Markets,” Subcontract ZAP-5-15299-02, Golden, Colo.)
Some of most promising early efforts to develop solar collector mirror panels used sandwich-type construction. In sandwich-type construction, membranes, such as sheet steel, aluminum, or plastic, are bonded to both sides of a core material. This type of construction is widely utilized in products ranging from doors and tables to aircraft and boats and is characterized by high strength-to-weight ratios. In the case of solar collectors, glass mirrors are adhesively bonded to one of the membranes. Examples of sandwich construction mirrors include the Solar One heliostat mirrors (Stone, et al., 1993), the Solar Kinetics, Inc. Innovative Concentrator Panels (Schertz, P. T., 1986, “Design of a Point-Focus Concentrator,” Proceedings of the Solar Thermal Technology Conference, Diver, R. B. (ed.), SAND86-0536, Sandia National Laboratories, Albuquerque, N. Mex.), the General Electric Parabolic Dish Concentrator (the PDC-1 used a reflective film) (Sobczak, I. F., Pons, R. L., Thostesen, T., 1982, “Development Status of The PDC-1 Parabolic Dish Concentrator,” Parabolic Dish Solar Thermal Power Annual Program Review Proceedings, DOE/JPL-1060-52, Pasadena, Calif.), and the Cummins Utility-Scale dish concentrator. Some of the early prototype trough mirrors also used sandwich-type construction mirrors.
Recently, stretched-membrane designs incorporating membranes of plastic or steel stretched over both sides of a ring have received a lot of attention. In the stretched membrane design, vacuum in the plenum between the membranes is used to create the required curvature. Examples include LaJet/Cummins panels, the SAIC USJVP dish and several heliostat designs. Bean, J. R., Diver, R. B., 1995, “Technical Status of the Dish/Stirling Joint Venture Program,” Proceedings of the 30th Intersociety Energy Conversion Engineering Conference, Paper Number 95-202, Orlando, Fla.; Beninga, K., Butler, B., Sandubrae, J., Walcott, K., 1989, “An Improved Design for Stretched-Membrane Heliostats,” SAND89-7027, Albuquerque, N. Mex.; and Beninga, K, Davenport, R. L., Sellars, J. A., Smith, D., Johansson, S., 1997, “Performance Results for the SAIC/STM Prototype Dish/Stirling System,” Proceedings of the 1997 ASME International Solar Energy Conference, Washington, D.C. Stretched-membrane solar collectors with plastically deformed metal membranes have been developed by Solar Kinetics, Inc. Schertz P. E., P. T., Brown, D. C. Konnerth III, A., 1991, “Facet Development for a Faceted Stretched-Membrane Dish by Solar Kinetics, Inc.,” SAND91-7009, Sandia National Laboratories, Albuquerque, N. Mex.; and Schlaich, J., Bergermann, R., Schiel, W., 1994, “Solar Stretch,” Civil Engineering. 
In spite of the considerable activity put toward the development of solar collectors, there remains the need for a cost effective, solar collector that is durable and has good optical properties.